Fix #233 crash on mkv with invalid video stream.
Fix #233 crash on mkv with invalid video stream.

mltframework/mlt,mltframework/mlt,j-b-m/mlt,xzhavilla/mlt,siddharudh/mlt,j-b-m/mlt,j-b-m/mlt,mltframework/mlt,j-b-m/mlt,mltframework/mlt,siddharudh/mlt,siddharudh/mlt,mltframework/mlt,j-b-m/mlt,siddharudh/mlt,j-b-m/mlt,siddharudh/mlt,j-b-m/mlt,mltframework/mlt,mltframework/mlt,xzhavilla/mlt,siddharudh/mlt,xzhavilla/mlt,xzhavilla/mlt,mltframework/mlt,xzhavilla/mlt,siddharudh/mlt,siddharudh/mlt,siddharudh/mlt,j-b-m/mlt,xzhavilla/mlt,xzhavilla/mlt,j-b-m/mlt,j-b-m/mlt,xzhavilla/mlt,mltframework/mlt,mltframework/mlt,xzhavilla/mlt